Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, and various signal processing circuits.
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual images for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The structure of semiconductor material allows the material's electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed operations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each semiconductor die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual semiconductor die from the finished wafer and packaging the die to provide structural support, electrical interconnect, and environmental isolation. The term “semiconductor die” as used herein refers to both the singular and plural form of the words, and accordingly, can refer to both a single semiconductor device and multiple semiconductor devices.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller semiconductor die size can be achieved by improvements in the front-end process resulting in semiconductor die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
As demand for smaller and higher performance semiconductor devices increases, various packaging types such as ball grid array (BGA), flip chip, and wafer level chip scale package (WLCSP) are being developed and refined. Recent demands for WLCSP designs include larger semiconductor die size, finer interconnect pitch, higher performance, and lower cost. Solder joint reliability becomes a challenge for finer pitch and higher input/output (I/O) density devices while also reducing the cost of manufacturing WLCSP.
Another goal of semiconductor manufacturing is to produce more reliable semiconductor devices. In a conventional WLCSP structure, a passivation layer is formed over contact pads on a semiconductor wafer. An extremely-low dielectric constant (ELK) insulating layer may be formed beneath the contact pads. Redistribution layers (RDL), passivation layers, and polymer layers are formed over the semiconductor wafer. An under-bump metallization (UBM) layer is formed over the RDL and passivation layers prior to forming a solder bump. Intermetallic compound (IMC) layer forms between the solder bump and UBM. A semiconductor die is mounted to a substrate by the solder bumps. IMC also forms between the solder bumps and contact pads on the substrate. Brittle materials, such as IMC and ELK layers, are particularly susceptible to cracking under stress during WLCSP manufacturing.
Electronic packaging material properties and geometrical dimensions impact the mechanical behavior of semiconductor packages. In many cases, excessive levels of stress imposed upon a conductive joint structure during the manufacturing process may cause failure phenomena such as UBM layer delamination, polymer layer delamination, IMC cracking, ELK cracking, solder bump cracking, and semiconductor die cracking which in turn reduces reliability and manufacturing yield. For example, delamination between the UBM layer and RDL is a common failure mode for a conventional WLCSP solder joint. Solder bump cracking is another common failure mode. The solder bump cracks near a corner of the RDL layer and propagates along the die-side IMC layer to cause joint failure. The solder bump also cracks near the contact pad on the substrate and propagates along the substrate-side IMC layer to cause joint failure. The use of a UBM layer reduces stress in the solder joint, however, the process of forming a UBM layer increases the cost of the WLCSP.